The blades of a rotorcraft rotor in hover have the same airspeed, angle of attack and lift coefficient as the rotor rotates. In forward flight the blade rotating in the direction of the flight (advancing blade) has the rotorcraft airspeed in addition to the airspeed due to the rotor angular velocity. The blade rotating opposed to the direction of the flight (retreating blade) has its airspeed due to rotor angular velocity reduced by the vehicle airspeed.
Rotors consist of aerodynamically shaped blades attached to a rotating mast at the center of rotation. In some rotors (teetering rotors) the blades are attached to a hub which is free to teeter one blade up, opposing blade down. In other rotors (articulated rotors) the hub is rigidly attached to the rotating mast and the blades are attached to the hub, at a point outboard of the center of rotation, by an articulated attachment allowing the blades to flap up and down. In other rotors (semi-rigid rotors) the blades are attached to the rotating mast through a flexible hub which allows the blades to flap up or down proportionally to up or down moment applied by the blade on the hub. The blades of rigid rotors are rigidly attached to the hub and the rotating mast in the up-down flap direction.
To allow for rotorcraft control and maneuver, the rotor blades are either supported on bearing or on elastic blade retention, both of these methods provide for controlling the pitch angle of the blade as it rotates. The pitch control axis at the root of the blade is called feather axis. Aerodynamic and inertia loads deflect the blade away from the feather axis.
The varying aerodynamic lift, drag and pitch moment on the individual blade as it rotates in forward flight create oscillatory loads on the rotor. The aerodynamic and dynamic characteristics of the rotor determine the dynamic response of the rotor to such loads; which, in turn, effect the movement of the blade and therefore the blade lift, drag and pitch moment. In the last 3 decades, developers have used sophisticated and complex computerized analysis to predict rotor loads, and to thereby drastically reduce rotorcraft vibration.
Nevertheless, some of the remaining rotor loads (especially pitch link loads) and vibration levels of rigid and semi-rigid rotors, are the result of prior art blades having been built “straight” along the axis of blade rotation in pitch, which is defined herein as the “feather axis”. The bending of such blades under lift, drag, and centrifugal forces, moves the blade away from the feather axis, thereby producing unnecessary vibration. The vibration in turn substantially affect passenger acceptance, crew fatigue, life of the rotor components, failure rate of the rotor system, empty weight of the rotorcraft, and the cost of operating the rotorcraft per flight hour.
In the prior art, proprotor and other rotor blades have been altered away (bent down) from the blade feather axis to improve aerodynamic performance and/or reduce rotor acoustic signature (noise level). But such modifications away from the feather axis have been always limited to about the outermost 10% of the radius from the rotor center of rotation, R90 and R80, respectively. Those limited modifications are not enough to reduce rotor loads and vibrations to anywhere near a minimum achievable amount. Thus, there is still a need for design and implementation of rotor blades and rotorcraft having reduced vibration.